Microstructural surface incorporation of phase change materials for thermal management

ABSTRACT

A process for protecting an article from thermal damage includes anodizing a metallic surface of the article to form an anodic layer containing a metal oxide; annealing the anodic layer; introducing a phase change material to pores defined by the anodic layer; and applying a seal layer to seal the phase change material within the pores.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 63/392,063, filed Jul. 25, 2022, which isincorporated by reference herein in its entirety.

BACKGROUND

Multiple systems and components operate at high power, high speed orunder circumstances that expose them to elevated temperatures. All ofthese conditions could compromise the functionality of the systems andcomponents. Thermal runaway in batteries, central processing unit (CPU)overheating, and ablation of materials are common examples of thenegative effects that high temperatures can cause. To protect equipmentfrom thermal damage, various types of thermal management systems havebeen developed, which can often be categorized into active and passivethermal management. Examples of active thermal management arerefrigeration systems that pump refrigerant to absorb heat from one sideof the system and expel heat at a different site. That is, the activethermal management system uses equipment to transport fluid that carriesheat from the component to the environment. Such is the case on somerefrigeration systems in DDG-51 class naval ships, which employ R-114refrigerant. Overall, active thermal management strategies are a veryeffective means of reducing temperature in a system. However, activethermal management requires different designs and additional powercompared to passive management approaches, resulting in higher operatingcosts. To cut down on energy costs, industries have been investing inpassive cooling systems, which employ materials and/or geometriesapplied to equipment to effectively exchange heat with the environmentwithout the need of a power source. Heat sinks, heat spreaders, and heatpipes are all examples of passive thermal management.

Phase change materials are another example of a passive thermalmanagement approach. Phase change materials absorb and release heat attemperatures at or near their phase change temperature. These phasechanges are endothermic while heating (when the material istransitioning from solid to liquid) and exothermic upon cooling (duringthe transformation from liquid back to solid). Furthermore, phase changematerials have been successfully employed by diverse industries, fromsolar, buildings/construction, to textiles, and electronics.

BRIEF DESCRIPTION

A fabrication route capable of generating porous structures withoutdamaging the host structure or reducing its mechanical properties isdisclosed herein. The desired micro channels or pores accept a phasechange material and be strong enough to sustain the various forcesplaced upon them. Moreover, the phase change materials will need to besealed to prevent leakage during the heating stages and be able tosustain multiple thermal cycles.

The concept of generating a porous anodic layer on the surface of ametallic component to host a phase change material (phase changematerials) is intended to reduce the peak temperatures that the hoststructure will experience.

Disclosed, in some embodiments, is a process for protecting an articlefrom thermal damage. The process includes anodizing a metallic surfaceof the article to form an anodic layer comprising a metal oxide;annealing the anodic layer; introducing a phase change material to poresdefined by the anodic layer; and applying a seal layer to seal the phasechange material within the pores. The metallic surface may containaluminum or an aluminum alloy. In some embodiments, the metallic surfaceis anodized with oxalic acid or sulfuric acid. The phase change materialmay contain n-eicosane. In some embodiments, the article is a heat sink,a casing, a fan, or a circuit board cooling device. Non-limitingexamples of casings include a pump casing, a transmission casing, adifferential casing, an electronics casing, and a power generation heatexchange casing.

Disclosed, in other embodiments, is an article including a metallicsubstrate; an anodic layer comprising a metal oxide formed on themetallic substrate and defining a plurality of pores; a phase changecomposition within the plurality of pores; and a seal layer sealing thephase change composition within the plurality of pores. The metallicsubstrate may contain aluminum or an aluminum alloy. In someembodiments, the phase change material includes n-eicosane. The articlemay be a heat sink.

Disclosed, in further embodiments, is a method for microstructuralsurface incorporation of phase change materials. The method includesanodizing a surface of an article including an aluminum alloy with anacid; annealing the aluminum alloy in air at a temperature below themelting point of the aluminum alloy; vacuum-impregnating the aluminumalloy with a phase change material; and applying a seal layer over thealuminum alloy and phase change material. The seal layer may include anepoxy resin or a silver paint. In some embodiments, the surface isanodized with oxalic acid or sulfuric acid. The phase change materialmay include n-eicosane.

These and other non-limiting characteristics are more particularlydescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 is a flow chart illustrating a method for producing a thermallyprotected article in accordance with some embodiments of the presentdisclosure.

FIG. 2 is a side cross-sectional view of a portion of a thermallyprotected composite in accordance with some embodiments of the presentdisclosure.

FIG. 3 includes SEM images of surface structures produced during a40-minute anodization treatment at different acid concentrations andvoltages as discussed in the Examples.

FIG. 4 includes SEM images of surface structures produced during a25-minute anodization treatment at different acid concentrations andvoltages as discussed in the Examples.

FIG. 5 includes pore size distribution graphs for different treatmentlengths as discussed in the Examples.

FIG. 6 is a temperature profile graph as discussed in the Examples.

FIG. 7 is a temperature different graph for a specimen utilizing anepoxy sealant as discussed in the Examples.

FIG. 8 is a temperature different graph for a specimen utilizing asilver paint sealant as discussed in the Examples.

DETAILED DESCRIPTION

The present disclosure may be understood more readily by reference tothe following detailed description of desired embodiments includedtherein, the drawings. In the following specification and the claimswhich follow, reference will be made to a number of terms which shall bedefined to have the following meanings.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art. In case of conflict, the present document, includingdefinitions, will control. Preferred methods and materials are describedbelow, although methods and materials similar or equivalent can be usedin practice or testing of the present disclosure. All publications,patent applications, patents, and other references mentioned herein areincorporated by reference in their entirety. The materials, methods, andarticles disclosed herein are illustrative only and not intended to belimiting.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used in the specification and in the claims, the term “comprising”may include the embodiments “consisting of” and “consisting essentiallyof.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,”“contain(s),” and variants thereof, as used herein, are intended to beopen-ended transitional phrases that require the presence of the namedingredients/steps and permit the presence of other ingredients/steps.However, such description should be construed as also describingcompositions, mixtures, or processes as “consisting of” and “consistingessentially of” the enumerated ingredients/steps, which allows thepresence of only the named ingredients/steps, along with any impuritiesthat might result therefrom, and excludes other ingredients/steps.

Unless indicated to the contrary, the numerical values in thespecification should be understood to include numerical values which arethe same when reduced to the same number of significant figures andnumerical values which differ from the stated value by less than theexperimental error of the conventional measurement technique of the typeused to determine the particular value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of “from 2 to 10” isinclusive of the endpoints, 2 and 10, and all the intermediate values).The endpoints of the ranges and any values disclosed herein are notlimited to the precise range or value; they are sufficiently impreciseto include values approximating these ranges and/or values.

As used herein, approximating language may be applied to modify anyquantitative representation that may vary without resulting in a changein the basic function to which it is related. Accordingly, a valuemodified by a term or terms, such as “about” and “substantially,” maynot be limited to the precise value specified, in some cases. Themodifier “about” should also be considered as disclosing the rangedefined by the absolute values of the two endpoints. For example, theexpression “from about 2 to about 4” also discloses the range “from 2 to4.” The term “about” may refer to plus or minus 10% of the indicatednumber. For example, “about 10%” may indicate a range of 9% to 11%, and“about 1” may mean from 0.9-1.1.

For the recitation of numeric ranges herein, each intervening numberthere between with the same degree of precision is explicitlycontemplated. For example, for the range of 6-9, the numbers 7 and 8 arecontemplated in addition to 6 and 9, and for the range 6.0-7.0, thenumber 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 areexplicitly contemplated.

FIG. 1 is a flow chart illustrating a process 100 in accordance withsome embodiments of the present disclosure. The process 100 includespretreating a metal substrate 110, anodizing the metal substrate 120,annealing the metal substrate 130, providing a phase change material topores 140, and applying a seal layer to seal in the phase changematerial 150.

FIG. 2 is a side cross-sectional view of a portion of thermallyprotected component 201 in accordance with some embodiments of thepresent disclosure. The component 201 includes a metal substrate 215, anoxide layer 225, a phase change material 235 provided in a pore, and aseal layer 245. Only one pore is illustrated. However, it should beunderstood that FIG. 2 illustrates only a portion of a component, and aplurality of pores will be present. Moreover, the depicted pore andother elements are not drawn to scale.

The metal substrate 215 may contain at least one metal element and/or atleast one metalloid element. The metal(s) may be selected from Li, Be,Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y,Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Cs, Ba, La, Ce, Pr, Nd, Pm,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au,Hg, TI, Pb, Bi, Po, Fr, Ra, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es,Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, FI, Mc, and Lv.The metalloid(s) may be selected from B, Si, Ge, As, Sb, and Te.

In particular embodiments, the metal substrate 215 comprises elementalaluminum or an aluminum alloy.

The aluminum alloy may be a 1000 series aluminum alloy, a 2000 seriesaluminum alloy, a 3000 series aluminum alloy, a 4000 series aluminumalloy, a 5000 series aluminum alloy, a 6000 series aluminum alloy, a7000 series aluminum alloy, an 8000 series aluminum alloy, or a 9000series aluminum alloy.

The aluminum content in the aluminum alloy may be at least 50 wt %, atleast wt %, at least 70 wt %, at least 80 wt %, at least 85 wt %, atleast 90 wt %, at least 91 wt %, at least 92 wt %, at least 93 wt %, atleast 94 wt %, at least 95 wt %, at least 96 wt %, at least 97 wt %, atleast 98 wt %, or at least 99 wt %.

In addition to aluminum, the aluminum alloy may contain one or moremetal or metalloid selected from the group consisting of Li, Be, Na, Mg,Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb,Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, TI,Pb, Bi, Po, Fr, Ra, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md,No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, FI, Mc, Lv, B, Si, Ge,As, Sb, and Te.

The substrate 215 may have a thickness in the range of about 0.1 μm toabout 900 μm, including from about 1 μm to about 800 μm and from about10 μm to about 700 μm.

Pretreatment 110 of the substrate may include cleaning. Cleaning mayutilize a sodium hydroxide solution in some embodiments. Although notspecifically illustrated in FIG. 1 , it is also contemplated thatcleaning may occur after one or more of the anodizing 120, annealing130, phase change material introduction 140, and/or seal layerapplication 150. When more than one cleaning step is utilized, the stepsmay be different as it is known in the art that different surfacecompositions may require different treatments to remove contaminants.For example, after anodization 120, the surface may be rinsed withdeionized water, ethanol, and dried.

The anodization 120 may utilize an acid. The acid may be provided in asolution (e.g., an aqueous solution). Non-limiting examples of acidsinclude sulfuric acid, oxalic acid, chromic acid, tartaric acid, citricacid, malic acid, boric acid, and phosphoric acid. Combinations of anytwo or more of the aforementioned acids may also be used.

The concentration of the acid may be in a range of about 0.1 M to about5 M, including from about 0.5 M to about 2.5 M, and about 1.2 M to about1.6 M.

The applied voltage during the anodization 120 may be in a range ofabout 10 V to about 150 V, including from about 20 V to about 100 V, andabout 30 V to about 40 V.

Anodization 120 may be performed for a time period of from about 1minute to about 90 minutes, including from about 5 minutes to about 60minutes, and from about minutes to about 40 minutes.

Temperature during anodization 120 may be in a range of about 20° C. toabout ° C., including from about 30° C. to about 50° C., and about 35°C. to about 45° C.

Anodization 130 may result in pore sizes (average or median) in a rangeof about 1 nm to about 500 nm, including from about 3 nm to about 200nm, from about 4 nm to about 100 nm, and from about 5 nm to about 85 nm.

Annealing 130 is intended to improve the mechanical robustness prior tothe introduction of the phase change material.

Annealing 130 may be conducted at a temperature in a range of from about400° C. to about 600° C., including from about 450° C. to about 550° C.,and about 475° C. to about 525° C.

In some embodiments, annealing 130 is performed for a time period offrom about 10 minutes to about 210 minutes, including from about 30minutes to about 180 minutes, and from about 90 minutes to about 150minutes.

The annealed article may be permitted to cool after annealing 130.

The phase change material may be introduced 140 via vacuum impregnation.

The phase change material 235 may be selected from waxes (e.g., paraffinwax), hydrocarbons (e.g., alkane hydrocarbons), fatty acids (e.g.,stearic acid), hydrated salts, and phase change alloys such asindium-bismuth-tin alloys (e.g., Field's alloy which contains about 51%indium, about 32.5% bismuth, and about 16.5% tin. Mixtures andcomposites are also contemplated. Non-limiting examples of alkanesinclude any one or more of n-octacosane, n-heptacosane, n-hexacosane,n-pentacosane, n-tetracosane, n-docosane, n-tricosane, n-heneicosane,n-eicosane, n-nonadecane, n-octadecane, n-heptadecane, n-hexadecane,n-pentadecane, n-tetradecane, and n-tridecane.

The phase change material may be introduced along with one or moreadditives. Non-limiting examples of suitable additives include bothnano-scale additives and/or micro-scale additives and, in someembodiments, may be selected from metals, metal oxides, and carbon-basedmaterials (e.g., graphene, carbon nanotubes).

Non-limiting examples of sealant materials 245 include epoxies andmetallic paints (e.g., silver paints). It is also contemplated thatstainless steel may be utilized for the seal layer.

The seal layer 245 may have a thickness in a range of about 0.005 mm toabout 1 mm, including from about 0.01 mm to about 0.05 mm and from about0.013 mm to about 0.018 mm.

The objective of the inclusion of the phase change material(s) is toreduce the peak temperatures that are observed when the metallic part issubjected to transient thermal loads. The reduced cross section of thesurface layer did not meaningfully impact the functionality of themetallic component nor negatively impacted its mechanical propertiesduring testing. The process disclosed herein could be applied togeometrically complex metallic substrates without the need for designand fabrication modifications, which cannot be achieved by theprocedures known in the art. The process can be applied as apost-processing step in complex metallic parts, and will minimallyimpact the existing design, weight, and overall mechanical properties.

The process could be applied to any metallic substrate that is subjectto thermal loads and is required to stay within a certain temperaturerange for optimal performance. Potential applications include but arenot limited to:

-   -   high power electronics;    -   micro-electronics;    -   heating ventilating and air conditioning heat (HVAC) exchange        systems;    -   machinery casings to include pump, transmission, and        differential casings;    -   power generation heat exchange casings;    -   3D printer heat exchanger fans;    -   electronics casings; and    -   circuit board cooling devices.

The process results in the incorporation of phase change materials intoa metallic part at the microstructural level through a simplepostproduction process that could be applied to existing, geometricallycomplex components.

Some advantages of the approach presented herein are the simplicity ofthe strategy, the small amounts of phase change materials required, andthat there is no need to redesign the components. Moreover, anodizationis a widely employed surface treatment and the process is easilyscalable. The mechanical properties of the original metallic part mayremain unchanged or relatively unchanged.

The following examples are provided to illustrate the devices andmethods of the present disclosure. The examples are merely illustrativeand are not intended to limit the disclosure to the materials,conditions, or process parameters set forth therein.

Examples

The conditions to fabricate a porous anodic layer on top of an aluminumsubstrate were determined through varying anodization conditions:solution concentration, voltage employed and anodization times. Poresizes were characterized using scanning electron microscopy. The alkanen-eicosane was selected as a phase change material, introduced withinthe porous anodic annealed layer using vacuum impregnation, and then thethin film composite structure was sealed. Epoxy resin and a metallicpaste were tested as sealants. Thermal tests were performed to comparethe behavior of aluminum alloy substrates anodized and sealed with andwithout phase change materials. The results showed that the aluminumalloy impregnated with n-eicosane presents lowered peak temperaturesduring heating cycles than the samples that were only anodized or thanthe base alloy, demonstrating the potential of phase change materialsincorporated in the superficial microstructure of anodic structures tomanage, to a certain extent, peak transient thermal loads.

An anodic layer was successfully fabricated on a given aluminum surfacefrom an existing part. By adjusting the variables of electrolyteconcentration, electrolyte temperature, time, and voltage, an anodiclayer that contained pores with an average diameter of 50 nm wereconsistently fabricated for use with various phase change materials andsealant methods.

The anodic layer was then annealed, a step that increased the mechanicalrobustness of the layer. Vacuum impregnation was successfully employedto incorporate phase change materials into the porous anodic structureand sealed with various coating agents. The vacuum impregnation chamberconstructed as part of this work was highly effective at creating avacuum seal. The metal-phase change materials composite generatedwithstood multiple heating cycles during the vacuum impregnation processwhich was critical to the fabrication of multiple phase change materialsincorporated anodic samples.

The results indicate that there is a reduction of 1 to 2.3° C. in thepeak temperatures that the metallic host experiences, with only 0.05grams of n-eicosane was spread throughout a 120 mm² section of aluminum,which is itself a remarkable result for a composite structure that onlymeasures a few microns height and contains pores in the nanometer scalelocated in the surface of the component.

Materials and Methods

Surface Layer Fabrication

Instead of starting with the full body of a heat sink, these examplesemployed an aluminum fin that was removed from a commercial heat sink toserve as proof of concept that could later be scaled up. The aluminumfin was anodized to create a porous layer, annealed at moderatetemperatures to strengthen the anodic structure, vacuum impregnated withn-eicosane phase change materials, and sealed with either epoxy resin orsilver paste.

The aluminum alloy sample to be anodized was employed as the anode whilethe cropped aluminum heat sink base of the same alloy was used as acathode. The cathode had a surface area that was significantly largerthan that of the anode (1:32 ratio). Electrical insulation tape was usedto cover the back and the edges of the aluminum fin anode, leavingapproximately 120 mm² of aluminum exposed to the electrolyte. A DC powersource (Model XLN15101 B&K Precision Corp, Yorba Linda, CA, US) anddigital multimeter (Model 2100 Keithley Instruments, Cleveland, OH, US)were placed in series and connected to metal clamps that held the anodeand cathode, the positive end being connected to the anode and thenegative end to the cathode. The power source was programmed to supply afixed amount of voltage over a set amount of time.

Concerning the anodization conditions, the method was adopted from astudy conducted by Sanz et al. (“Aluminum Anodization in Oxalic Acid:Controlling the Texture of Al₂O₃/Al Monoliths for CatalyticApplications,” Ind. Eng. Chem. Res., vol. 50, no. 4, pp. 2117-2125, Feb.2011, DOI: 10.1021/ie102122x which is incorporated by reference hereinin its entirety). For the purposes of this study, well-structured openpores of micrometer scale length were sought after to host the phasechange materials. The aluminum was anodized at 1.2 and 1.6 M oxalic acid(98%, Sigma-Aldrich, Burlington, MA, US) concentrations, using voltagesof 20, 30, 40 and 100 V, with a fixed value of 40° C. for the bathtemperature. The experiments were performed during time intervals thatspanned between 10 and 40 minutes.

After the creation of the anodic layer, the tape employed to limit thearea exposed to the bath was removed from the fin samples, which werethen rinsed with DI water, ethanol and dried. The specimens were placedin an oven at 500° C. for 2 hours (process herein referred as anneal)and allowed to gradually cool.

During the vacuum impregnation process, the non-anodic sections of thealuminum fin were again covered in tape, and 0.05 g of solid n-Eicosane(Millipore Sigma, Saint Louis, MO, US) was placed on top of the anodiclayer. The fin was then placed in a hermetically sealed dish that wasconnected to a vacuum pump (roughing pump that achieves 1×10⁻³ torr) andplaced into a binder convection oven (Binder GmbH, Tuttlingen, Germany).Once the vacuum pump was activated, the oven was set to 45° C. for thephase change materials to melt. The intent of placing the liquid phasechange materials below atmospheric pressure was to force the phasechange materials to fill the vertically oriented pores that theanodization step produced. Samples were allowed to cool while undervacuum.

To prevent phase change material leakage during the heating and coolingcycles, each sample was sealed by the application of a surface coat.Diverse sealing materials were compared based on their availability,ease of application, thermal stability, and on the ability to containthe phase change materials after several heating cycles. To prepare eachsample, the outer rim of the aluminum fin was washed with ethanol toensure that all tape residue from the vacuum impregnation process hadbeen removed. The non-anodic surface was then sanded with 300 gritsandpaper to ensure that the chosen sealant had a strong mechanical bondto the area surrounding the phase change material.

Epoxy resin was used as one of the methods of encapsulation. Epofixresin (Struers, Ballerup, Denmark), with a 25 to 3 resin to hardenerratio by mass, was chosen for the first experiment. The resin-hardenermixture was stirred for five minutes and allowed to settle for fiveadditional minutes. Then a thin coat of epoxy was applied with a finehaired brush. Once the fin had the uncured epoxy applied, it was placedin the same vacuum chamber used for impregnation to remove air bubblesintroduced from mixing the epoxy components. Once the vacuum was appliedusing the roughing pump, the epoxy system was allowed to dry at roomtemperature for 24 hours.

The second sealant used, here referred to as silver paint, was composedof silver particles dispersed in Iso-Butyl Methyl Ketone (Ted Pella Inc.Redding, CA, US). To distribute the silver particles prior toapplication, the silver paint bottle was placed in an ultrasonic cleanerfor ten minutes. After covering the fin with silver paint, the aluminumfin was placed in the vacuum chamber for thirty minutes. While thealuminum fin was drying in the vacuum chamber, the silver paint wasplaced in the sonication bath to ensure that any particles displacedafter the first application were again evenly distributed. After thirtyminutes, a second coat of silver paint was applied to the entire surfaceand placed in the vacuum chamber for thirty minutes, after which thevacuum pump was turned off and the fin was left to dry for 24 hours.

The porous structures produced were observed employing scanning electronmicroscopy (SEM). A Zeiss Neon 40 (Carl Zeiss Inc., Thornwood, NY, USA)field emission SEM and a FEI Inspect 50 SEM (Field Electron and IonCompany, Hillboro, OR, US), both operating between 1 and 20 KV were usedto characterize the pore sizes and anodic layer height. The softwareprogram Image J (National Institutes of Health, Bethesda, Maryland, US)was used for the statistical analysis of pore sizes.

Once the phase change material was sealed, thermal testing was conductedusing a FLIR USETS320 thermal camera (FLIR Systems Inc. San Carlos, CA,US). The thermal test was constructed by placing a Pyrex dish filledwith sand on top of a hot plate and positioning the samples on top ofthe sand. The FLIR camera was set 65 mm above the samples surface. Thesand bed was used to slow the heating rate, allowing the measurement ofthe local temperatures within each sample. A bare aluminum fin, alonganodized-annealed-sealed fins with, and without phase change materials,were tested simultaneously. The samples were sealed under identicalconditions to match their emissivity. Heating and cooling cycles wererun, and the temperature plotted with respect to time for each sample.

Visual observation of the fin samples showed a yellow deposit afteranodization, which turned brown after thermal treatments at 500° C. Themelting and dispersion of the phase change materials during vacuumimpregnation left a thin film that was later coated. The silver sealantwas observed for the specimen utilizing silver paint. Other specimensshowed a transparent coat (epoxy resin) that allowed the observation ofthe underlying structure.

The pores generated by diverse anodization conditions are presented inFIGS. 3 and 4 . Those SEM images show evidence of regular arrangement ofpores under most conditions employed. All samples in FIG. 3 werefabricated by experiments that lasted 40 min at 40° C. in theirrespective concentrations. Larger pore volumes are observed for samplesgenerated at 40 V when compared to those produced at 20 or 30 V. Whenlooking at the oxalic acid concentration of 1.6 M, it was observed thathigher voltages created larger diameter pore sizes in the anodicstructure. For the application of phase change material impregnation,these larger pore sizes were preferred. Since the largest pore sizeshown in FIG. 3 is observed at 40 V, such voltage and concentration werechosen to move forward with for the anodic layer development.

With the voltage and concentration of the solution fixed at 40 V and 1.6M respectively, time would not only determine the thickness of theanodic layer, but also the pore morphology. FIG. 4 details the effectsof anodization times: after 25 min under the mentioned experimentalconditions, a more disordered and misoriented group of pores isgenerated. With an increase in porosity comes a thinning of the cellwalls that surround each anodic pore, increasing the likelihood ofcolumns that bend and break, blocking the anodic pores and rendering amicrostructure ineffective for phase change materials impregnation andreduced ability to hold the phase change materials during thermalcycling.

A more detailed analysis of the effects of time in the pore sizedistribution is presented in FIG. 5 , where it can be observed that mostpores have diameters in the 5-85 nm range, with average values thatincrease as the time of anodization is extended.

Comparing these results with studies done in the past with anodicstructures produced with oxalic acid, it is clear that there is a pointin which overgrowth could occur. As temperature, current density, andelectrolyte concentration increase, the rate at which the pores formincreases. As time continues to advance, the formation of the anodiclayer becomes thicker, while the diameters of the pores will continue togrow larger. Those pores will eventually experience overgrowth orchalking. Chalking is due to a chemical attack at the outer part of theoxide film which thins the pore walls and causes their upper regions tolose structural stability and collapse.

Thermal treatment at 500° C. did not cause significant changes in themicrostructure of the samples, or in morphology and size since SEMobservations rendered diameters within the standard deviation of themeasurements taken before annealing. The vacuum impregnation of thephase change materials in the anodic structures was followed by theapplication of a surface coat.

FIG. 6 shows the temperature profiles of samples tested with and withoutphase change materials. The phase change experienced by the phase changematerials produced the expected endothermic reaction during phase changematerials melting (M.P. 36-38° C.), and an exothermic reaction duringsolidification. Upon heating, this endothermic reaction slowed the rateof heating a significant amount. This effect lasted for approximately4.88 minutes starting at a temperature of 37.06° C. and ending at atemperature of 39.02° C. This resulted in a maximum temperaturedifference of 2.27° C. when compared with the anodic layer with an epoxycoat and no phase change materials, and a maximum temperature differenceof 1° C. when compared with raw aluminum. Upon cooling, the exothermicreaction peaked at 36.96° C., which is below the lower bounds of thephase change materials liquid phase transition temperature. Duringtesting, however, the phase change materials leaked out of the epoxycoating, reducing the effectiveness of the sample to withstand repeatedcycles.

The test performed with phase change materials sealed with two thincoats of silver paint presented a similar cooling effect than the oneobserved with Epofix. However, the silver paint coating was able tocontain the phase change materials upon heating and cooling, a promisingresult regarding repeatability. The maximum relative temperaturedifference 1.63° C. when compared with the anodic layer with the silvercoat and no phase change materials, and a maximum temperature differenceof 3.35° C. when compared with raw aluminum.

FIGS. 7 and 8 are graphs showing the temperature differences betweenmetallic samples that had an anodic structure that was annealed andsealed and one that had the annealed anodic structure containing phasechange materials and sealed with Epofix coat (FIG. 7 ) or silver paintcoat (FIG. 8 ) along with the images of the respective samples. Largerdifferences between the samples with and without phase change materialsare detected for samples sealed with epoxy resin than those sealed withsilver paint, although the latter seems to maintain the lowertemperature during longer periods of time.

Other literature that encapsulates phase change materials in heatspreaders or similar structures reports larger reductions in peaktemperatures than those observed here. However, in those cases, themetallic component employed had to be redesigned to host the phasechange materials. From the observed results, it can be said that greaterdistribution of phase change materials with a conductor's surface areaallows the phase change materials to efficiently absorb heat from theconductor resulting in a net reduction of surface temperatures between 1and 2.3° C. with only 0.05 grams of n-eicosane was spread throughout a120 mm² section of aluminum. Phase change materials within the anodiclayer containing features in the nanometer scale increased the surfacearea contact of the phase change materials-conductor interface. As aproof of concept, these examples proved that the efficient applicationof the phase change materials within the microstructure can decrease thepeak transient thermal loads of the system into which the phase changematerial is integrated. Given the results presented herein, it isbelieved that latent heat energy storage could be achieved in morecompact passive heat management devices, paving the way fortechnological advancement in the field of thermal management.Furthermore, phase change materials incorporated within the surfacemicrostructure of components could be applied not only to electronicdevices, but to machinery and other systems that operate under cyclicloads.

This written description uses examples to describe the disclosure,including the best mode, and also to enable any person skilled in theart to make and use the disclosure. Other examples that occur to thoseskilled in the art are intended to be within the scope of the presentdisclosure if they have structural elements that do not differ from thesame concept, or if they include equivalent structural elements withinsubstantial differences. It will be appreciated that variants of theabove-disclosed and other features and functions, or alternativesthereof, may be combined into many other different systems orapplications. Various presently unforeseen or unanticipatedalternatives, modifications, variations, or improvements therein may besubsequently made by those skilled in the art which are also intended tobe encompassed by the following claims.

1. A process for protecting an article from thermal damage, the processcomprising in sequence: anodizing a metallic surface of the article toform an anodic layer comprising a metal oxide; annealing the anodiclayer; introducing a phase change material to pores defined by theanodic layer; and applying a seal layer to seal the phase changematerial within the pores.
 2. The process of claim 1, wherein themetallic surface comprises aluminum.
 3. The process of claim 1, whereinthe metallic surface comprises an aluminum alloy.
 4. The process ofclaim 1, wherein the metallic surface is anodized with oxalic acid. 5.The process of claim 1, wherein the metallic surface is anodized withsulfuric acid.
 6. The process of claim 1, wherein the phase changematerial comprises n-eicosane.
 7. The process of claim 1, wherein thearticle comprises a heat sink.
 8. The process of claim 1, wherein thearticle comprises a casing, a fan, or a circuit board cooling device. 9.The process of claim 8, wherein the casing is selected from the groupconsisting of a pump casing, a transmission casing, a differentialcasing, an electronics casing, and a power generation heat exchangecasing.
 10. An article comprising: a metallic substrate; an anodic layercomprising a metal oxide formed on the metallic substrate and defining aplurality of pores; a phase change composition within the plurality ofpores; and a seal layer sealing the phase change composition within theplurality of pores.
 11. The article of claim 10, wherein the metallicsubstrate comprises aluminum.
 12. The article of claim 10, wherein themetallic substrate comprises an aluminum alloy.
 13. The article of claim10, wherein the phase change material comprises n-eicosane.
 14. Thearticle of claim 10, wherein the article comprises a heat sink.
 15. Amethod for microstructural surface incorporation of phase changematerials, the method comprising: anodizing a surface of an articlecomprising an aluminum alloy with an acid; annealing the aluminum alloyin air at a temperature below the melting point of the aluminum alloy;vacuum-impregnating the aluminum alloy with a phase change material; andapplying a seal layer over the aluminum alloy and phase change material.16. The method of claim 15, wherein the seal layer comprises an epoxyresin.
 17. The method of claim 15, wherein the seal layer comprisessilver paint.
 18. The method of claim 15, wherein the surface isanodized with oxalic acid.
 19. The method of claim 15, wherein thesurface is anodized with sulfuric acid.
 20. The method of claim 15,wherein the phase change material comprises n-eicosane.